Figure 2.

Information contained in the full 4D measurement of biphoton joint probability distribution. (a–c) Variation of ${\mathrm{\Gamma }}_{i|j}$ at different distances from the center, indicated by blue ×, showing anti-correlation of width that increases with |x| (see insets). (d) Projection of ${\mathrm{\Gamma }}_{ij}$ onto sum coordinates averages the variations in (a–c). (e–g) 2D slices of ${\mathrm{\Gamma }}_{ij}$ for fixed $[x_i,x_j]$ (indicated by blue dashed lines in inset of 〈$C_i$〉) showing variation in anti-correlation with horizontal separation. (h) Projection of ${\mathrm{\Gamma }}_{ij}$ onto $[y_i,y_j]$ (integration over $x_i$ and $x_j$) averages the structures in (e–g), giving only a mean profile.